Solid-State Discrete-Frequency Synthesizer

This article describes the effective utilization of nonprecise tuning components in unique techniques for precise frequency synthesis. Frequency synthesis is the generation of sinusoidal RF signals of precisely controlled and accurately known arbitrary frequencies. Nonprecise components which are examined in detail under the several limiting conditions of operation are solid-state devices, such as variable-capacitance diodes, ferroelectric capacitors, and controllable inductors. The significance of combining a discrete-frequency reference and and an electronically-tuned phase-lock oscillator in the generation of precise frequencies is that it is possible to construct a unit whose output frequency can have certain discrete values. This unit, called a discrete-frequency generator, has an output frequency which is precisely one of the harmonics of the discrete-frequency reference. By using several discrete-frequency generators, each with different incremental steps, combining and taking the mixed output, it is possible to cover a wide over-all range in small incremental steps. The design and construction of a discrete-frequency synthesizer which tunes over a 1-Mc range in 10-kc steps (two significant figures) is carried out to demonstrate the practicability of the proposed technique. The discrete-frequency reference is provided by a five-stage shift-register generator. The shift-register generator is synchronized by a crystal clock which has a long-term frequency stability of one part in 10⁵. The spurious output of the synthesizer is at least 50 db below the desired output. The significant consequences of this investigation result from the extreme flexibility afforded by completely electronic tuning methods.